Large xenotime ceramic block and process for making the same

ABSTRACT

A ceramic block consisting essentially of substantially homogeneous (Y 2 O 3 ) x .P 2 O 5 , where 0.95≦x≦1.05, having length LL, a width WW and a height HH, and a volume VV of at least 8×10 −3  m 3  essentially free of cracks throughout the volume, a density of at least 85% of the theoretical maximal density of Y 2 O 3 .P 2 O 5  under standard conditions, and a creep rate at 1250° C. and 6.89 MPa of CR, where CR≦8.0×10 −6  hour −1 , and method for making the same. The method utilizes a dry process where the starting YPO 4 -based ceramic material is synthesized by reacting anhydrous P 2 O 5  with dry Y 2 O 3  powder.

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application No. 61/408,071 filed on Oct. 29, 2010 thecontent of which is relied upon and incorporated herein by reference inits entirety.

TECHNICAL FIELD

The present invention relates to YPO₄-based ceramic materials andprocess for making such materials. In particular, the present inventionrelates to large YPO₄-based ceramic blocks having low levels ofcontaminants, low creep rate at elevated temperatures and processes formaking such blocks. The present invention is useful, e.g., in makingisopipes used in the overflow down-draw processes for making glass sheetsuitable for LCD glass substrates.

BACKGROUND

Fusion down-draw is a leading precision technology developed by CorningIncorporated, Corning, N.Y., U.S.A. for making thin glass sheetssuitable for use as liquid crystal display (LCD) glass substrates and inother opto-electronic devices. This process is schematically illustratedin FIG. 1. A stream of molten glass is introduced into a forming trough103 called isopipe with end-caps 105 at both ends and having two sidesurfaces converging at a line called root 109 via an inlet pipe 101coupled to the trough of the isopipe. The glass melt is then allowed tooverflow both top surfaces of the trough of the isopipe called weirs,flow down along both side surfaces of the isopipe as two molten glassribbons 107, then join and fuse at the root 109 to form a single glassribbon 111, which is then drawn down in the direction 113 and cooledbelow the root to form the glass sheet with desired dimensions. In thezone below the root, the glass ribbon travels substantially verticallydownward while being drawn and cooled from a viscous state, tovisco-elastic and eventually to substantially elastic. The elastic glassribbon is then cut into individual glass sheets, subjected to furtherfinishing such as edge rounding and polishing, and then packaged andshipped to LCD panel makers for use as TFT or color filter substrates.Cutting of the glass ribbon at below the isopipe typically involves thescoring of the ribbon surface, followed by bending along the score-line,whereby discrete glass sheets are separated from the ribbon and thentransferred to subsequent steps.

One of the advantages of the fusion down-draw process for making glasssheets is that the surface quality of the glass sheets is high becausethe quality areas thereof were only formed in the atmosphere and nevertouched a solid material such as the forming equipment. This process hasbeen used successfully for making glass sheets having a width as largeas 3000 mm and a thickness of about 0.6 mm.

The average size of LCDs for the consumer electronics market hasincreased steadily in the past decade, along with the demand for higherimage quality. These have fueled the demand of large-width glasssubstrates and posed increasingly more stringent requirements for glasssheet quality, such as edge warp and waviness, sheet warp, surfacewaviness and roughness, thickness uniformity, as well as stress.

At the center of the overflow down-draw process is the isopipe. Thedimension and dimension stability of the isopipe has significant impacton the dimension and dimension stability of the glass sheet formed. Theisopipe is typically made of a refractory block of material such aszircon ceramics. While the isopipe is supported on both ends, it istypically not supported in the middle. At the high operatingtemperatures and under the heavy load of the gravity of the isopipe andthe glass melt inside the trough and on the surfaces, the isopipe issubject to slow deformation due to a physical phenomenon calledcreeping. The higher the creep rate of the material of the isopipe, themore the isopipe can creep over a given period of time. In addition, theisopipe material is desirably stable and corrosion-resistance withrespect to the glass melt it handles. While zircon was found acceptablefor making LCD glass substrates for smaller generation glass sheets, ithas relatively high creep rate for even larger generation isopipes, suchas those having a length of over 3000 mm. In addition, zircon was foundto be less than ideal in corrosion-resistance for some glasscompositions.

YPO₄-based ceramic materials were proposed for isopipes previously.However, making large-size ceramic materials based on YPO₄ is not aneasy undertaking. Therefore there remains a need of a large ceramicblock based on YPO₄ suitable for an isopipe and method for making thesame. The present invention satisfies this and other needs.

SUMMARY

Several aspects of the present invention are disclosed herein. It is tobe understood that these aspects may or may not overlap with oneanother. Thus, part of one aspect may fall within the scope of anotheraspect, and vice versa.

Each aspect is illustrated by a number of embodiments, which, in turn,can include one or more specific embodiments. It is to be understoodthat the embodiments may or may not overlap with each other. Thus, partof one embodiment, or specific embodiments thereof, may or may not fallwithin the ambit of another embodiment, or specific embodiments thereof,and vice versa.

Thus, a first aspect of the present invention relates to a ceramic blockconsisting essentially of substantially homogeneous (Y₂O₃)_(x).P₂O₅,where 0.95≦x≦1.05, having a length LL, a width WW and a height HH, and avolume VV of at least 8×10⁻³ m³ essentially free of cracks throughoutthe volume, a density of at least 85% of the theoretical maximal densityof Y₂O₃.P₂O₅ under standard conditions, and a creep rate at 1250° C. and6.89 MPa (1000 psi) of CR, where CR≦8.0×10⁻⁶ hour⁻¹, in certainembodiments CR≦7.0×10⁻⁶ hour⁻¹, in certain embodiments CR≦6.0×10⁻⁶hour⁻¹, in certain embodiments CR≦5.0×10⁻⁶ hour⁻¹, in certainembodiments CR≦4.0×10⁻⁶ hour⁻¹, in certain embodiments CR≦3.0×10⁻⁶hour⁻¹, in certain embodiments CR≦2.0×10⁻⁶ hour⁻¹.

In certain embodiments of the first aspect of the present invention,1.00≦x≦1.05, in certain embodiments 1.00≦x≦1.03, in certain otherembodiments 1.00≦x≦1.02.

In certain embodiments of the first aspect of the present invention,LL≧20 cm, WW≧20 cm, and H≧20 cm.

In certain embodiments of the first aspect of the present invention,LL≧50 cm, WW≧30 cm, and HH≧50 cm.

In certain embodiments of the first aspect of the present invention,LL≧100 cm, WW≧30 cm, and HH≧50 cm.

In certain embodiments of the first aspect of the present invention,LL≧200 cm, WW≧30 cm, and HH≧50 cm.

In certain embodiments of the first aspect of the present invention,LL≧300 cm, WW≧30 cm, and HH≧50 cm.

In certain embodiments of the first aspect of the present invention, theceramic block comprises calcium at a concentration by weight of [Ca],where [Ca]≦100 ppm, in certain embodiments [Ca]≦80 ppm, in certainembodiments [Ca]≦50 ppm, in certain embodiments [Ca]≦40 ppm.

In certain embodiments of the first aspect of the present invention, theceramic block comprises zirconium at a concentration by weight of [Zr],where [Zr]≦50 ppm, in certain embodiments [Zr]≦40 ppm, in certainembodiments [Zr]≦30 ppm, in certain embodiments [Zr]≦20 ppm, in certainembodiments [Zr]≦10 ppm.

In certain embodiments of the first aspect of the present invention, theceramic block comprises aluminum at a concentration by weight of [Al],where [Al]≦60 ppm, in certain embodiments [Al]≦50 ppm, in certainembodiments [Al]≦40 ppm, in certain embodiments [Al]≦30 ppm, in certainembodiments [Al]≦20 ppm.

In certain embodiments of the first aspect of the present invention, theceramic block is essentially free of alkali metal.

In certain embodiments of the first aspect of the present invention, theceramic block comprises barium at a concentration by weight of [Ba],where [Ba]≦100 ppm, in certain embodiments [Ba]≦80 ppm, in certainembodiments [Ba]≦50 ppm, in certain embodiments [Ba]≦40 ppm.

In certain embodiments of the first aspect of the present invention, theceramic block is essentially free of carbon.

A second aspect of the present invention relates to a method for makinga ceramic block consisting essentially of substantially homogeneous(Y₂O₃)_(x).P₂O₅, where 0.95≦x≦1.05, having a length LL, a width WW and aheight HH, and a volume VV of at least 8×10⁻³ m³, essentially free ofcracks throughout the volume, a density of at least 95% of thetheoretical maximal density of Y₂O₃.P₂O₅ under standard conditions, anda creep rate at 1250° C. and 6.89 MPa of CR, where CR≦8.0×10⁻⁶ hour⁻¹,comprising the following steps:

(I) providing a dry powder of Y₂O₃ and a dry powder of P₂O₅;

(II) mixing the powder of Y₂O₃ and the powder of P₂O₅ to obtain auniform powder mixture;

(III) sintering the powder mixture obtained in step (II) in a firstfurnace to obtain a precursor ceramic material having a composition(Y₂O₃)_(x).P₂O₅;

(IV) crushing the precursor ceramic material obtained in step (III) toobtain a plurality of particles;

(V) forming a first green body having a volume larger than VV, of theceramic block by mixing uniformly the plurality of particles obtained instep (IV) with an organic binder;

(VI) isopressing the first green body obtained in step (V) at a pressureof at least 500 MPa to obtain a second green body having a density atstandard conditions of at least 60% of the theoretical maximal densityof Y₂O₃.P₂O₅ under standard conditions; and

(VII) heating the second green body in a second furnace to a temperatureof at least 1500° C.

In certain embodiments of the second aspect of the present invention,the method comprises a step (VIII) after step (VII):

(VIII) removing the material within 1.0 cm from the surface of theceramic block.

In certain embodiments of the second aspect of the present invention,step (VII) comprises:

(VII.1) increasing the temperature of the furnace from 200° C. to 1500°C. at an average temperature elevation rate of not higher than 50°C./hour, in certain embodiments not higher than 40° C./hour, in certainembodiments not higher than 30° C./hour, in certain embodiments nothigher than 20° C./hour, in certain embodiments not higher than 10°C./hour; and

(VII.2) maintaining the temperature of the furnace at over 1500° C. forat least 100 hours, in certain embodiments at least 200 hours, incertain embodiments at least 300 hours, in certain embodiments at least400 hours, in certain embodiments at least 500 hours.

In certain embodiments of the second aspect of the present invention, instep (VII), the furnace environment in the second furnace is oxidizingand essentially free of alkali metals.

In certain embodiments of the second aspect of the present invention, insteps (I), (II) and (V), contamination by Al, Ba, Ca and Zr are avoided.

In certain embodiments of the second aspect of the present invention, instep (V), the organic binder is used at an amount of at most 5% byweight of the second green body.

In certain embodiments of the second aspect of the present invention,step (V) comprises the following steps:

(V-1) providing the plurality of particles in the following separatecomponents in the following respective amounts:

(p1) from 15%-35% of particles having a particle size not higher than 45μm and a median particle size about 2 μm;

(p2) from 15% to 35% of particles having a particle size not higher than45 μm and a median particle size about 5 μm; and

(p3) from 45% to 65% of particles having a particle size from 45 μm to100 μm;

(V-2) mixing the three components (p1), (p2) and (p3) with a binderuniformly to obtain an admixture; and

(V-3) forming the first green body from the admixture obtained in step(V-2).

In certain embodiments of the second aspect of the present invention,step (IV) comprises:

(IV-1) pressing the precursor ceramic obtained in step (III) into largeparticles;

(IV-2) grinding at least part of the large particles obtained from step(IV-1) into fine particles; and

(IV-3) passing the fine particles obtained in step (IV-2) through amagnetic separator to remove metal particles.

In certain embodiments of the second aspect of the present invention, insteps (I), (II), (IV), (V) and (VII), direct contact of the materialbeing handled with CaO-containing, BaO-containing or ZrO₂-containingmaterial is avoided.

One or more embodiments and/or aspects of the present disclosure haveone or more of the following advantages. First, the large-sizeYPO₄-based ceramic block can be made essentially free of Second, due tothe low concentrations of trace metal contaminants, the ceramic blockexhibits an exceedingly low level of creep rate at the high operatingtemperatures, which can be particularly advantageous for makinglarge-size isopipes, such as those having a length of at least 2000 mm,in certain embodiments at least 3000 mm. Third, owing to the use of thedry synthesis approach, the ceramic block can be made essentially freeof water, resulting in excellent creep rate at high temperatures.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from the description or recognizedby practicing the invention as described in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that the foregoing general description and thefollowing detailed description are merely exemplary of the invention,and are intended to provide an overview or framework to understandingthe nature and character of the invention as it is claimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic illustration of an isopipe in operation in anoverflow down-draw process for making a glass sheet.

FIG. 2 is a schematic illustration of a cross-section of the isopipeshown in FIG. 1.

FIG. 3 is a diagram showing the relationship between the creep rate at1180° C. and 6.89 MPa of a series of ceramic materials and the sum totalof the concentration of the metal contaminants.

FIG. 4 is a diagram showing the relationship between the creep rate at1180° C. and 6.89 MPa of a series of ceramic materials and [Ca] and [Zr]thereof.

FIG. 5 is a diagram showing the temperature profiles of two examplesaccording to the present invention in the final sintering step and themeasured dynamic shrinkage of the second green bodies during thesintering step.

DETAILED DESCRIPTION

Unless otherwise indicated, all numbers such as those expressing weightpercents and mole percents of ingredients, dimensions, and values forcertain physical properties used in the specification and claims are tobe understood as being modified in all instances by the term “about.” Itshould also be understood that the precise numerical values used in thespecification and claims form additional embodiments of the invention.Efforts have been made to ensure the accuracy of the numerical valuesdisclosed in the Examples. Any measured numerical value, however, caninherently contain certain errors resulting from the standard deviationfound in its respective measuring technique.

As used herein, in describing and claiming the present invention, theuse of the indefinite article “a” or “an” means “at least one,” andshould not be limited to “only one” unless explicitly indicated to thecontrary. Thus, for example, reference to “a metal oxide” includesembodiments having one, two or more such metal oxides, unless thecontext clearly indicates otherwise.

In the present application, the terms xenotime, YPO₄ and Y₂O₃.P₂O₅ areused interchangeably to mean a yttrium phosphate material. A YPO₄-basedmaterial is a material comprising primarily of YPO₄, with or withoutexcess amount of Y₂O₃ or P₂O₅ over the stiochiometry required for YPO4,and other minor components as may be found therein.

As used herein, the term (Y₂O₃)x.P₂O₅ means a yttrium phosphate materialcomprising Y₂O₃ and P₂O₅ wherein the molar ratio of Y₂O₃ to P₂O₅ is x.

In the present application, the first furnace and the second furnace maybe the same or different.

As used herein, the concentration of a given element X, [X], such [Na],is the concentration of the elemental metal relative to the totalcomposition by weight. All percentages and ppm as used herein are byweight, unless specified otherwise.

The overflow down-draw process for making a sheet glass is described inWO03/014032, WO05/081888, and the like, the relevant contents thereofare incorporated herein by reference in their entirety.

FIGS. 1 and 2 schematically illustrate an isopipe assembly 100 duringnormal operation. The isopipe comprises an upper trough-shaped part 102and a lower wedge-shaped part 104, which, in combination, form a unitaryforming body 100. The trough-shaped part comprises a first internaltrough side surface 121, a second internal trough side surface 123 and atrough bottom surface 122, which together define an open channel (alsocalled “trough”) 103 into which the glass melt is introduced, typicallythrough an open end thereof. The glass melt is allowed to overflow thefirst trough top surface 125 of the first trough wall and the secondtrough top surface 127 of the second trough wall, flow down along afirst external trough side surface 129 and a second external trough sidesurface 131, and further down along a sloping first wedge side surface133 connecting with the first external trough side surface 129, and asloping second wedge side surface 135 connecting with the secondexternal trough side surface 131. At the root 109 where the two wedgeside surfaces 133 and 135 join, the two glass ribbons fuse to form aunitary glass ribbon 111, which is further drawn down in the direction113 to a desired thickness, cooled down to elastic state, and then cutinto individual glass sheet pieces with desired size. For the purpose ofconvenient description, a virtual plane 199 having the smallestcumulative difference in distances to (I) the first internal trough sidesurface 121 of the first trough wall and (II) the second internal troughside surface 123 of the second trough wall is defined as the centerplane of the open channel (trough). Thus, if the open channel issymmetrical with respect to a plane passing through the centerline ofthe trough, the center plane would be the plane with respect to whichthe two sides of the open channel are symmetrical. Desirably, both thetrough-shaped part and the wedge-shaped part are symmetrical withrespect to the center plane of the open channel. In such scenario, thecenter plane 199 would also pass through the root line 109 of theisopipe.

WO06/073841 and WO09/108298 disclose YPO₄-based materials and processesfor making them, as well as the use of such materials for isopipes, thecontents of both are incorporated herein by reference in their entirety.The isopipe is the center of an overflow down-draw process for makingglass sheet. Thus its geometric stability is very important for makingglass sheets with consistent thickness and thickness variation over along production campaign. Since the isopipe is a long object typicallyoperating at an elevated temperature, e.g., at about 1200° C. forprolonged periods of time while supported only at the ends, the isopipeis subjected to creeping, i.e., geometric deformation due to the weightof the isopipe and the glass melt it contains. Stringent requirementsare imposed on the refractory materials for making the isopipe. Suchrequirements are well documented in the prior art references such asWO06/073841 and WO09/108298 mentioned supra.

A large, continuous and unitary block of ceramic material havingsubstantially homogeneous composition and physical properties throughoutits volume is desired to fabricate a single isopipe forming body.Hitherto no YPO₄-based ceramic block having a volume of over 5×10⁻³ m³with suitable composition and properties for an isopipe has beenreported. This is due primarily to the difficulties in fabricating suchlarge ceramic blocks with such stringent property requirements,especially a low creep rate at an elevated temperature such as 1250° C.Given the refractoriness of YPO₄, the fabrication of such block wouldnecessarily require a relatively long step of high-temperaturetreatment. Further, a plurality of steps are required during each ofwhich a number of factors would affect the final composition,composition homogeneity, contamination level, microstructure, stressdistribution, and the like, hence the final composition and propertiesof the ceramic block.

Through extensive research and development effort, the present inventorshave successfully made large YPO₄-based ceramic blocks having a volumeof at least 8×10⁻³ m³, low level of contaminants and propertiesespecially creep rate at 1250° C. that meets the requirements of anisopipe for making large-size glass sheets. Both the large ceramic blockand the process for making such ceramic blocks, among others,constitutes the various aspects of the present invention. The ceramicblock is characterized by a creep rate at 1250° C. and 6.89 MPa of CR,where CR≦8.0×10⁻⁶ hour⁻¹, in certain embodiments CR≦7.0×10⁻⁶ hour⁻¹, incertain embodiments CR≦6.0×10⁻⁶ hour⁻¹, in certain embodimentsCR≦5.0×10⁻⁶ hour⁻¹, in certain embodiments CR≦4.0×10⁻⁶ hour⁻¹, incertain embodiments CR≦3.0×10⁻⁶ hour⁻¹, in certain embodimentsCR≦2.0×10⁻⁶ hour⁻¹.

The chemical composition of the ceramic block is substantially uniform,i.e., the major components Y₂O₃ and P₂O₅ are distributed substantiallyuniformly throughout the full volume of the bulk. Thus, the materialconstituting the ceramic block may comprise essentially a single phase,such as the YPO₄ phase, through the bulk in certain embodiments.However, it is possible that the material may comprise multiple phasesthat are all substantially uniformly distributed inside the bulk. Forexample, the material constituting the ceramic block may comprise, inaddition to a main YPO₄ phase, a minor Y₂O₃ phase distributedsubstantially uniformly in the YPO₄ phase. Trace amounts of impurities,as mentioned infra, such as Al₂O₃, BaO, B₂O₃, CaO, MgO, MnO, ZrO₂, andthe like, may be present in the bulk at various amounts. Since they aretypically present at a very low concentrations, as described supra, thedistribution thereof may exhibit an irregular pattern. For example, theconcentration of sodium, [Na], may be higher in the vicinity of thesurface of the ceramic block, and lower in regions far from the surfaceof the block, due to the high-temperature diffusion of sodium infurnaces from the surface to the center.

The chemical composition of the ceramic block may be represented by aformula (Y₂O₃)_(x).P₂O₅, where x is the molar ratio between Y₂O₃ andP₂O₅, and 0.95≦x≦1.05. Thus the material may be a stoichiometric YPO₄material, or may comprise excess amount of Y₂O₃ or P₂O₅. Nonetheless, itis more advantageous that the Y₂O₃ molar amount is not lower than P₂O₅,thus it is desired that 1.00≦x≦1.05, in certain embodiments 1.00≦x≦1.03,in certain other embodiments 1.00≦x≦1.02. This is because excessive P₂O₅can lower the melting temperature of YPO₄ faster than the same amount ofY₂O₃.

The dimensions of the ceramic block can be described in terms of lengthLL, width WW and height HH. In certain embodiments, the blockadvantageously has a large size where LL≧20 cm, WW≧20 cm, and HH≧20 cm.The block can be a full solid block having a cubic, cuboidal, spherical,spheroidal, or other geometry. The block may take the shape of anisopie, i.e., one comprising a trough-shaped top part and a wedge-shapedbottom part connected with each other. The block can advantageously havea shape where LL≧50 cm, WW≧30 cm, and HH≧50 cm. For larger size isopies,the block may have a length LL≧100 cm, in certain embodiments LL≧200 cm,in certain other embodiments LL≧300 cm.

One of the interesting findings in the extensive research anddevelopment efforts by the present inventors is the low contaminantconcentrations, especially [Ca], [Zr] and [Al], in the ceramic body ofthe present invention. Such low levels of contamination, among otherfactors, gave rise to the low creep rate at 1250° C. and 6.89 MPa. Incertain embodiments, the ceramic block of the present inventionadvantageously comprises calcium at a concentration by weight of [Ca],where [Ca]≦100 ppm, in certain embodiments [Ca]≦80 ppm, in certainembodiments [Ca]≦50 ppm, in certain embodiments [Ca]≦40 ppm. In certainembodiments, the ceramic block of the present invention advantageouslycomprises zirconium at a concentration by weight of [Zr], where [Zr]≦50ppm, in certain embodiments [Zr]≦40 ppm, in certain embodiments [Zr]≦30ppm, in certain embodiments [Zr]≦20 ppm, in certain embodiments [Zr]≦10ppm. In certain embodiments, the ceramic block of the present inventionadvantageously comprises aluminum at a concentration by weight of [Al],where [Al]≦60 ppm, in certain embodiments [Al]≦50 ppm, in certainembodiments [Al]≦40 ppm, in certain embodiments [Al]≦30 ppm, in certainembodiments [Al]≦20 ppm. In certain embodiments, the ceramic block ofthe present invention advantageously comprises barium at a concentrationby weight of [Ba], where [Ba]≦100 ppm, in certain embodiments [Ba]≦80ppm, in certain embodiments [Ba]≦50 ppm, in certain embodiments [Ba]≦40ppm. In certain embodiments, the ceramic block of the present inventionis advantageously characterized by the sum total of [Al], [B], [Ba],[Ca], [Fe], [Hf], [K], [Li], [Mg], [Mn], [Na] and [Zr] being at most 500ppm by weight, in certain embodiments at most 400 ppm by weight, incertain other embodiments at most 300 ppm by weight, in certain otherembodiment at most 200 ppm by weight, in certain other embodiments atmost 100 ppm by weight. Making a large YPO₄-based ceramic body havingsuch low levels of contaminants is by no means an easy undertaking dueto the multi-step process required, the large volume of materials thathave to be handled, and the high temperature steps needed. On the otherhand, the successful synthesis of a ceramic block with such low levelsof concentration of contaminants resulted in the excellent propertiesthereof, especially the low creep rate at high temperatures such as1250° C. at 6.89 MPa.

In addition to the lack of metal contaminants, the refractory block ofthe present invention is further advantageously characterized by a lowlevel of carbon in certain embodiments. Carbon can be entrapped in anydense material made by sintering because organic matters can beintroduced into the material prior to firing, either unintentionally dueto material handling issues or intentionally because organic binders areused widely in processes for making them. The existence of carbon canaffect the mechanical properties of the ceramic block of the presentinvention, and causes undesired outgassing during normal use thereof.

The ceramic block of the present invention can be advantageously usedfor handling alkali-free glass materials aimed for applications inopto-electronic devices, such as the glass substrates for a LCD display.For example, the ceramic block can be formed into a large-size isopipeused in overflow down-draw processes for making aluminosilicateglass-based LCD glass substrates. For these applications, it is highlydesired that the ceramic block is essentially free of alkali metal ions,because such ions are particularly detrimental to semiconductormanufacture processes which will be conducted on the glass substrates.By “essentially free” as used herein is meant that the ceramic blockcomprises, by weight, at most 5 ppm of any alkali metal. In certainembodiments, the ceramic block of the present invention advantageouslycomprises at most 3 ppm, even more advantageously at most 1 ppm, of anyalkali metal.

The ceramic block of the present invention advantageously exhibits anominal density of at least 85%, in certain embodiments at least 87%, incertain embodiments at least 89%, in certain embodiments at least 90%,in certain embodiments at least 93%, in certain embodiments at least95%, in certain embodiments at least 97%, of the theoretical density ofYPO₄ under standard conditions (1 atmosphere pressure, 0° C.). Thetheoretical density of YPO₄ under standard conditions is typicallyconsidered as being 4.27 g·cm⁻³. The higher the density of the ceramicblock, the lower the porosity thereof. Given that the ceramic blockcomprises multiple grains, a certain level of porosity is expected toexist in the grain boundaries. The porosity at the grain boundary cansignificantly affect the mechanical properties of the ceramic block,such as, e.g., creep rate at an elevated temperature and modulus ofrupture (MOR) at room temperature and at the elevated operatingtemperature.

The ceramic block of the present invention further exhibits a low levelof cracks in the crystalline grains inside the bulk. It is believed thatthe existence of a high level of micro-cracks in the micro-structure ofthe ceramic block can reduce the MOR of the material and creep rateunder operating conditions. Such micro-cracks can propagate under loadand stress to a critical dimension, leading to material failure.

The ceramic block of the present invention is further characterized by alow water content. Water in the ceramic block of the present inventioncan take various forms, e.g., free water present in H₂O form trapped atthe grain boundary, or in the form of OH bonded to the surface or bulkof the material and/or the crystal grains. Without intending to be boundby a particular theory, it is believed that the existence of waterinside the bulk of the material can cause the formation of micro-cracksand propagation thereof under high operating temperatures. Therefore, itis desired that the total H₂O content in the ceramic block in certainembodiments be at most 300 ppm by weight, in certain other embodimentsat most 200 ppm by weight, in certain other embodiments at most 100 ppmby weight, in certain other embodiments at most 50 ppm by weight.

The ceramic block of the present invention having the advantageouscomposition and properties enumerated supra can be made by a carefullycontrolled synthesis method of the present invention. It is believedthat such large, high-purity, high-performance ceramic material cannotbe found in nature, and therefore has to be synthesized. As mentionedsupra, due to the high melting point of YPO₄, a high-temperature stepwould be unavoidable to make the ceramic block of the present invention.On the other hand, due to the high melting point of YPO₄, directlyforming such large block by melting the precursor material into a fluidfollowed by cooling as is typically used in forming glass materials andsome crystalline materials would be impractical. Thus, the presentinvention utilizes a sintering method, i.e., by heating a plurality ofdensely packed precursor particles having the intended final compositioninto a densified ceramic body containing multiple crystal grains.

As mentioned supra, the existence of large amount of H₂O in the ceramicblock of the present invention can lead to compromised mechanicalproperties. Therefore, the synthesizing method of the present inventionfor making the large-size block includes a first step of forming aprecursor ceramic material using a “dry” synthesis approach, i.e., byreacting anhydrous P₂O₅ with dry Y₂O₃ in the desired amounts to form theprecursor material with the desired end composition. It is to beunderstood that during the reaction of the first step, to expedite thereaction, it is desired that the reactants are heated to an elevatedtemperature such as at least 300° C., in certain embodiments at least400° C., in certain other embodiments at least 500° C., in certain otherembodiments at least 600° C. To reduce volatility loss of P₂O₅, it isdesired that the two powders are intimately mixed to form asubstantially homogenous mixture before being heated to an elevatedtemperature, and that the reaction is conducted in a substantiallyclosed container to prevent leakage of P₂O₅ gas. Moreover, due to thehighly hygroscopic nature of P₂O₅, it is desired that P₂O₅ is handled ina substantially dry atmosphere and the exposure thereof to the ambientis limited to as short a time as required only. To achieve a low levelof metal contaminants, it is desired that raw materials of Y₂O₃ and P₂O₅are as pure as possible, and the handling of both raw materials do notintroduce the unwanted metals. This requires the use of very cleancontainers made of materials that are substantially inert to Y₂O₃ andP₂O₅ even at the elevated reaction temperatures.

The present inventors conducted the synthesis of a large-size YPO₄-basedceramic block using various methods, including using precursor(Y₂O₃)_(x).P₂O₅ particles made by a “wet” synthesis approach, i.e., byreacting Y₂O₃ or precursor materials thereof (such as Y(OH)₃) withliquid H₃PO₄ or a solution thereof to obtain a precursor ceramicmaterial. Even if very pure raw materials were used and the handlingthereof was carefully controlled to avoid the introduction ofcontaminants, all other conditions being equal, a large-size YPO₄-basedceramic block with a low level of creep rate at 1250° C. essentiallyfree of visible cracks could not be successfully made using the wetsynthesis approach. Without intending to be bound by a particulartheory, the present inventors believes such failure of the “wet”synthesis approach was due in part to the large amount of waterintroduced into the precursor ceramic material particles, which isdetrimental to the final properties of the sintered ceramic block.

Upon complete sintering in step (III), the Y₂O₃ and P₂O₅ raw materialare substantially completely reacted to form a network material whereboth components are substantially evenly distributed throughout theprecursor material. However, such precursor material typically does nothave the required porosity and mechanical properties for use in anisopipe. Hence there is the need of subsequent steps (IV), (V), (VI) and(VII).

To obtain the final ceramic block of the present invention exhibiting alow level of metal contaminants, it is desired that in steps (I), (II),(IV), (V) and (VII), direct contact of the material being handled withAl₂O₃-containing, CaO-containing, BaO-containing or ZrO₂-containingmaterial is avoided.

Thus, for example, in step (IV), when the precursor ceramic material iscrushed to make the particles, it is desired that a grinding medium freeof Al₂O₃, CaO, and BaO is used.

The crushed particles made in step (IV) is typically separated intomultiple fractions having differing mean particle sizes, which, with orwithout further, separate, grinding, will be mixed at different amountsin step (V) in order to obtain a first green body having the desiredparticle packing, particle density and green body strength.

To obtain a strength of the first green body that can withstand thesubsequent handling, an organic binder is typically used. Such organicbinder may include, e.g., poly(meth)acrylates, METHOCEL, and the like.As mentioned supra, the use of organic binders in the green body canresult in a residual level of carbon in the final ceramic block, whichcan be detrimental to the final sintering step (VII) and the finalmechanical properties of the ceramic block. Thus, it is desired that thetotal amount of binders used in step (V) is at most 5% by weight of thefirst green body, in certain embodiments at most 3%, in certain otherembodiments at most 2%, in certain other embodiments at most 1%.

It was found that the nominal density of the second green body formed instep (VI) affects the final nominal density of the ceramic block uponcompletion of step (VII), and hence the mechanical properties thereof.It is desired that the density of the second green body is at least 60%of the theoretical density of YPO₄ crystals under standard conditions,in certain embodiments at least 63%, in certain other embodiments atleast 65%, in certain other embodiments at most 67%, in certain otherembodiments at least 69%, in certain other embodiments at least 70%.

Dense packing of the particles in steps (V) and (VI) are needed toachieve a high nominal density of the first and second green bodies. Toachieve a dense packing, the particles are desired to have a particledistribution facilitating packing thereof. To that end, in step (V), theparticles can comprise the following components:

(p1) from 15%-35% of particles having a particle size not higher than 45μm and a median particle size about 2 μm;

(p2) from 15% to 35% of particles having a particle size not higher than45 μm and a median particle size about 5 μm; and

(p3) from 45% to 65% of particles having a particle size from 45 μm to100 μm.

In an exemplary process, a large bulk of the precursor ceramic materialis crushed by using a metal plate into large particles, and then siftedinto three parts, a first part having particle size not lower than 100μm, a second part having a particle size not higher than 100 μm but notlower than 45 μm, and a third part having particle size not higher than45 μm. The second part is used as p3 above. The third part is thendivided into two subparts, Subpart I and Subpart II. Subpart I isfurther ground to have a median particle size of 2 μm and used as p1;and Subpart II is further ground to have a median particle size of 5 μmand used as p2. The existence and amount of the fine component p1,medium component p2 and the coarse component p3, above, can ensure asubstantially dense packing of the particles in step (VI). Furthermore,this distribution would result in a desirable crystal grain sizedistribution in the final ceramic block, which is desired for theintended MOR and creep rate and other mechanical properties of anisopipe.

The particles with the desired particle size distribution and amountsand the binder material are then mixed thoroughly and uniformly witheach other, put into a flexible bag, which can be vacuumed and shaken toallow compaction, and then sealed for isopressing in step (VI).

In step (VI), the first green body is subject to a high pressureisopressing process. The purpose of this step includes, inter alia,densifying the first green body to obtain a second green body with ahigh nominal density, and to achieve an isotropic density profilethroughout the second green body. An anisotropic density profile of thesecond green body would be detrimental to the final properties of thefinal ceramic block, as an isotropic density profile of the finalceramic block would be highly desired for the final ceramic block aswell. The isopressing is typically carried out in a machine calledisopress which comprises a container and a fluid that can be pressurizedto as high as 124 MPa (18000 psi). The first green body in a sealed bagis then placed into the fluid, and pressurized from all directions to apressure of at least 50 MPa, in certain embodiments at least 80 MPa, incertain other embodiments at least 100 MPa, in certain other embodimentsat least 120 MPa.

The temperature profile of the final sintering step (VII) significantlyimpacts the final properties and composition of the ceramic block. Ifthe temperature rises too fast, a high thermal gradient will occur fromthe surface to the core of the second green body, resulting insubstantial thermal stress that could crack the second green body beforethe end of step (VII). In addition, too steep a temperature rising curvecan cause undesirable early-stage closure of the pores in the vicinityof the surface, resulting in the entrapment of water, carbon, carbonmonoxide and carbon dioxide inside the ceramic body, causing undesiredproperties and/or later-stage cracking due to pressure built-up fromthese gaseous species. Therefore, it is desired that step (VII)comprises the following:

(VII.1) increasing the temperature of the furnace from 200° C. to 1500°C. at an average temperature elevation rate of not higher than 50°C./hour, in certain embodiments not higher than 40° C./hour, in certainembodiments not higher than 30° C./hour, in certain embodiments nothigher than 20° C./hour, in certain embodiments not higher than 10°C./hour; and

(VII.2) maintaining the temperature of the furnace at over 1500° C. forat least 100 hours, in certain embodiments at least 200 hours, incertain embodiments at least 300 hours, in certain embodiments at least400 hours, in certain embodiments at least 500 hours.

Thus, in step (VII.1), due to the slow temperature elevation rate, thesecond green body is subjected to a low temperature gradient from thesurface to the core, and it is allowed to degas through the open poreswhich close slowly, reducing the entrapment of water and carbon-basedcontaminants as well as the chance of cracking due to thermal gradient.In order to remove the organic materials substantially completely, it isdesired that at least in the initial stage of step (VII), the sinteringis carried out in an oxidizing environment, such as air, so that theorganic materials can be oxidized substantially completely and removedfrom the bulk of the green body before the full consolidation thereof.

During the final step of sintering, the second green body densifies byeliminating the porosities. Such densification is manifested by thereduction of volume, i.e., shrinkage. The higher the nominal density ofthe second green body, the lower the total shrinkage of the second greenbody at the end of the sintering step to achieve a given final densityof the ceramic block. The initial speed of shrinking is largelydetermined by the temperature of the green body. The higher the initialtemperature, the faster the shrinking speed the second green body willsee. However, the higher the initial furnace temperature, the morelikely a large temperature gradient from the surface to the core of thesecond green body will result. As mentioned supra, a slow temperatureelevation rate, especially at the beginning stage of the sintering step,is beneficial to reducing detrimental thermal gradient, and to reducingthe initial shrinking speed of the second green body. Therefore, theamount of shrinkage as well as the shrinking speed both can impact thefinal porosity, density and properties of the ceramic block.

To accommodate the shrinkage of the second green body during sintering,which can be quite significant for a large second green body, it isdesired that the second green body is positioned on a set up ofseparated rollers with or without fixed axles. Thus, when the surface ofthe second green body moves as a result of the bulk shrinkage, therollers will roll to accommodate the movement of the surface, therebyreducing the drag and frictional force exerted to the green body,allowing a complete densification without undesirable externalinterferences. As mentioned supra, during the sintering step, it isdesirable that the second green body is free from direct contact withcontaminant-containing materials. To that end, the rollers may becovered, e.g., by a layer of platinum as a barrier against migration ofcontaminants to the surface thereof. In general, to avoid the formationof cracks and to obtain desirable creep rate and other mechanicalproperties, the initial linear shrinkage speed of the second green bodyin a direction substantially perpendicular to the gravity vector isadvantageously controlled to less than 10 mm/hour, in certainembodiments less than 8 mm/hour, in certain other embodiments less than5 mm/hour, in certain other embodiments less than 3 mm/hour, during thefirst 100 hours of the heating process of the second green body to raiseits temperature from 200° C. to 1500° C. Further, the initial linearshrinkage speed of the second green body in a direction substantiallyperpendicular to the gravity vector is advantageously controlled to lessthan 5%/hour, in certain embodiments less than 4%/hour, in certain otherembodiments less than 3%/hour, in certain other embodiments less than2%/hour, during the first 100 hours of the heating process of the secondgreen body to raise its temperature from 200° C. to 1500° C.

Due to the refractoriness of the ceramic material used for the ceramicblock of the present invention, it is desired that in step (VII), thesecond green body is held at a temperature higher than 1600° C. for aperiod of at least 24 hours, in certain embodiments at least 72 hours,in certain embodiments at least 144 hours, in certain other embodimentsat least 240 hours, in certain other embodiments at least 360 hours, andstill in certain other embodiments at least 480 hours to allow forcomplete densification and the desired final density.

At the end of the sintering step (VII), after the second green body hasbeen substantially completely densified, it is allowed to cool down at alow cooling rate, e.g., not over 200° C./hour from 1600° C. to 200° C.,in certain embodiments advantageously not over 100° C./hour, in certainother embodiments advantageously not over 50° C./hour, in certain otherembodiments advantageous not over 10° C./hour, so that the cooling doesnot result in a substantial thermal gradient inside the bulk of theceramic block, which can cause cracking. It is also desirable that,after being cooled down to room temperature, the skin layer of theceramic block, having a depth of 1 cm, or 2 cm, or even 3 cm, beremoved, as the skin layer can comprise a disproportionately high levelof alkali metals such as sodium, potassium, and the like.

The present invention will be further illustrated by the followingnon-limiting examples and comparative examples.

Examples

A plurality of YPO₄-based ceramic rods were made and characterized forproperties. All examples, made from stoichiometric batch materials, arebelieved to have chemical compositions equal or close to stoichiometricYPO₄. Examples 7 and 8 were made by the dry process using anhydrous P₂O₅and Y₂O₃ as the raw materials for synthesizing the precursor ceramicmaterial. Examples 1, 2, 3, 4, 5, 6, 9 and 10 were made by using a wetprocess, i.e., using Y₂O₃ and H₃PO₄ as the raw materials forsynthesizing the precursor ceramic material.

The rods were then measured for creep rate (CR) at 1180° C. and 6.89MPa, and CR at 1250° C. at 6.89 MPa, respectively. Data of the creeprates are reported in TABLE I, below.

TABLE I Example No. 1 2 3 4 5 6 7 8 9 10 CR @ Series 1 2.82 2.62 2.991.82 2.41 3.13 0.918 1.03 4.13 5.83 1180° C. Series 2 3.07 4.67 4.323.76 3.65 3.96 0.899 0.853 4.27 6.69 (×10⁻⁷ Average 2.94 3.64 3.66 2.793.03 3.54 0.908 0.942 4.20 6.26 hour⁻¹) CR @ 1250° C. 11.1 14.1 12.39.38 7.39 10.8 1.69 1.21 37.4 38.0 (×10⁻⁷ hour⁻¹)

The rods where further measured of concentrations of contaminant metals.The results are reported in TABLE II, below.

TABLE II Example No. [Al] [B] [Ba] [Ca] [Fe] [Hf] [K] [Li] [Mg] [Mn][Na] [Zr] 1 75 5 3 130 67 0.6 <5 2 12 <1 11 30 2 60 2 3 120 71 2 <5 2 13<1 4 80 3 53 2 3 100 64 1 <5 2 19 <1 6 42 4 54 2 4 110 66 3.5 <5 2 14 <14 130 5 66 <1 2 130 77 0.6 <5 3 13 <1 14 29 6 79 <1 4 140 31 1.9 <5 3 15<1 29 95 7 19 <1 <1 10 18 0.5 <5 3 3 <1 4 29 8 20 <1 <1 11 <1 0.1 <5 2 3<1 7 9 9 87 <1 6 140 95 7.5 <5 2 28 <1 26 350 10 200 <1 2 120 32 9.9 <51 35 <1 21 450

As is clear, Example Nos. 7 and 8, both of the present invention,exhibit the lowest level of creep rates at both 1180° C. and 1250° C.,and the lowest levels of [Al], [Ba], [Ca], [Mg] and [Zr]. A diagramshowing the relationship between the creep rate at 1180° C. and 6.89 MPaand the sum total of the concentrations of contaminants[Al]+[B]+[Ba]+[Ca]+[Fe]+[Hf]+[K]+[Li]+[Mg]+[Mn]+[Na]+[Zr] is shown inFIG. 3. Shown on the vertical axis is the sum total of theconcentrations of these metals, and on the horizontal axis the creeprate at 1180° C. This figure clearly shows that the creep rate has astrong positive correlation with the sum total of the concentrations ofthese contaminants. In FIG. 4, the relationship between [Ca] and [Zr]and the creep rate at 1180° C. for these examples are shown with curve401 and the left vertical axis show the impact of [Ca], and curve 403and the right vertical axis show that of [Zr]. The data points with thelowest creep rates in both FIGS. 3 and 4 are for Example Nos. 7 and 8,described supra.

Based on the above learnings, a number of additional examples accordingto the present invention, i.e., using the “dry” approach to synthesizethe precursor ceramic material but with slightly different particledistribution of the particles used for making the first and second greenbody. The details the particle distributions of these examples areprovided in TABLE III below:

TABLE III Relative Composition of batch mixture green density ExampleComponent 1 Component 2 Component 3 of the second No. (%) (%) (%) greenbody (%) 11 45 25 30 68.8 12 45 30 25 69.2 13 50 35 15 69.8 14 50 30 2069.8 15 50 25 25 69.5 16 55 25 20 69.7 17 55 20 25 70.7

In above TABLE III, component 1 has a particle size of from 45-100 μm,component 2 has a median particle size of about 5 μm, and component 3has a mean particle size of about 2 μm, where all particles of thecomponents 2 and 3 are smaller than 45 μm. Experiments showed thatexamples 14 and 17 had the highest performance in terms of creep rate at1180° C. The materials were then used for making the green body oflarge-size ceramic blocks having a size of about 50 cm×50 cm×20 cm.

FIG. 5 shows the sintering temperature profiles of Examples 14 and 17.

It will be apparent to those skilled in the art that variousmodifications and alterations can be made to the present inventionwithout departing from the scope and spirit of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A ceramic block consisting essentially of substantially homogeneous(Y₂O₃)_(x).P₂O₅, where 0.95≦x≦1.05, having length LL, a width WW and aheight HH, and a volume VV of at least 8×10⁻³ m³ essentially free ofcracks throughout the volume, a density of at least 85% of thetheoretical maximal density of Y₂O₃.P₂O₅ under standard conditions, anda creep rate at 1250° C. and 6.89 MPa of CR, where CR≦8.0×10⁻⁶ hour⁻¹.2. A ceramic block according to claim 1 wherein 1.00≦x≦1.05.
 3. Aceramic block according to claim 1, wherein LL≧20 cm, WW≧20 cm, andHH≧20 cm.
 4. A ceramic block according to claim 1, wherein LL≧50 cm,WW≧30 cm, HH≧50 cm.
 5. A ceramic block according to claim 1, comprisingcalcium at a concentration by weight of [Ca], where [Ca]≦100 ppm.
 6. Aceramic block according to claim 1, comprising zirconium at aconcentration by weight of [Zr], where [Zr]≦50 ppm.
 7. A ceramic blockaccording to claim 1, comprising aluminum at a concentration by weightof [Al], where [Al]≦60 ppm.
 8. A ceramic block according to claim 1essentially free of alkali metal.
 9. A ceramic block according to claim1, comprising barium at a concentration by weight of [Ba], where[Ba]≦100 ppm.
 10. A ceramic block according to claim 1 essentially freeof carbon. 11-21. (canceled)